[MUSIC] Hello everyone, welcome back to my Coursera class, Biochemical Principles of Energy Metabolism. We are in the middle of photosynthesis. This is session three. Again, we are looking at a diagram showing the photosynthesis, which is composed of two reactions. First one is light-dependent pathway, the other one is Calvin cycle. In the previous session, we studied about how light energy can be used to drive the energy generation and electron donors. And on top of this, water become oxidized, and then oxygen can be released. So during this session, we are going to study about Calvin cycle, and we are going to basically complete the photosynthesis. So, Calvin cycle, it's about glucose production. It's time to fix inorganic carbons from CO2 into organic compound glucose. So Calvin cycle, or the other word, carbon fixation, is about ATP and NADPH consumption. And those precious materials produced from light-dependent reaction, we studied about that in the previous session. And those free energy and electrons are used to reduce inorganic CO2 into high energy containing organic compounds. And this reaction is not relying on the light energy. That's why this is called the light-independent reaction, or sometimes just say dark reaction. There are three steps of Calvin cycle. Number 1 is carbon fixation, reduction, and regeneration of the cycle. This is how it is, this is the Calvin cycle. The Stage number 1 is about CO2 molecules from the air, fixed into C3, three-carbon organic compound. That's why this is so-called C3 photosynthesis. Because the first product out of this carbon fixation is three-carbon based, C3 cycle, C3 photosynthesis. And then this is the number 1, carbon fixation. And second stage is reduction, reduction. So this 3-phosphoglycerate compound is further reduced into glyceraldehyde 3-phosphate. As you can see that this diagram, this carboxylic acid becomes reduced into aldehyde, right? And we need energy, and we need electrons. As you can imagine, these ATP and these expensive electron donors produced from light-dependent reaction, light-dependent reactions, okay? And then this glyceraldehyde 3-phosphate just go into a series of reactions, and finally, 6-carbon glucose can be generated. And to make sure this cycle is just being continuously operated, there is a step about regeneration of this initial substrate for carbon fixation, which is about ribulose bisphosphate. This is the Calvin cycle. So let's get into the step number 1, the carbon fixation. Carbon fixation, well, literally, this is the step for, how can I say, for the synthesis of glucose, high energy containing molecules, which are basically the sources for life on this planet. And this reaction is mediated by, catalyzed by, Rubisco. The name of enzyme is Rubisco, ribulose 1,5-biphosphate carboxylase/oxygenase. Because plant cells, they have this enzyme. In terms of number and mass, this is the uppermost abundant enzyme on this planet. But interesting thing is, this enzyme, the catalytic power, the Kcat, catalysis, the efficiency's very, very low, only 3 molecules per second. To compensate this slow biochemical reaction, plant cells produce huge amount of this protein, Rubisco enzymes. In terms of numbers, 10 to the 11th tons of CO2 fixed per year to sustain life on this planet. This is the chemical reaction. The incoming substrate is this one, ribulose 1,5-bisphosphate, simply speaking, C5 and 2 phosphate that you can see. 2phosphate, C5, and 1 carbon from the air can be fixed and then split into C3 compound. This is the most critical reaction for the Calvin cycle. And the second step is just the reduction, reduction of those C3 metabolites produced from step number 1, carbon fixation. In this case, we have to use energies, and we need electrons to reduce, right, to reduce the carbons. So as I said, this carboxylic acid, oxidized form, is further reduced into aldehyde form. And basically, this is the reversal reactions which are studied in the glycolysis. If you have some extra time, you can just review the glycolysis biochemical reactions. So I'm not going to explain the step number 3. But basically, once the first product, 3-PGA, produced and then further reduced into glyceraldehyde 3-phosphate. And these C3 compound's further condensed to produce finally a glucose molecule. In this reaction, to produce 1 glucose molecules, 6 CO2 supposed to be fixed, and then 18 ATP and 12 NADPH are consumed. That means energetically, this is very, very expensive pathway. But the thing is, throughout this reaction, plant cells can magically fix inorganic carbon into organic compound glucose. And human beings and many heterotrophs can utilize this glucose. Regeneration pathway is very straightforward. To make sure this cycle becomes continuously working, we have to regenerate out of this C3 compound, part of them reutilize to drive the synthesis of substrate RuBP. Okay, so once glucose molecules are produced out of Calvin cycle, plant cells should store that monomeric monosaccharide glucose in the form of polysaccharide, in this case, starch. 8 glucose molecules, ADP modified and activated and continuously polymerized throughout glycosidic bond. And then starch can be synthesized and stored, and this reaction occurs inside the chloroplast. In addition to starch, there is sucrose, and sucrose is a disaccharide composed of glucose and fructose. And those two sugars can be linked throughout the glycosidic bond, and then sucrose 6-phosphate, I mean, sucrose can be synthesized. And this reaction occurs, takes place in the cytoplasm of plant cells. That's the fate of hexose glucose after the Calvin cycle, starch or sucrose, and any other sugar molecules. I'm going to wrap up today's session, it's about Calvin cycle or C3 photosynthesis. Atmospheric inorganic CO2 can be fixed throughout the amazing enzyme called the Rubisco. And then the C3 compound, after carbon fixation, further reduced. And then reduced glyceraldehyde 3-phosphate, part of them can be used to redrive this cycle. And part of them can be used to generate final product, C6 glucose molecules, which is about carbohydrate, which are essential for sustaining life as a major energy source.
